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Waveguides Contain Spinning Photons

Photonics.comApr 2013
LONDON, April 24, 2013 — Electromagnetic waves in waveguides can now be guided in one direction by forcing photons to interact with circulating plasmon polaritons in the near field, or very close to the surface of a small metal nanostructure, an international study has found.

On the nanoscale, interactions of tiny electric fields created by charged particles can have intriguing effects on light's movements. These effects often occur through interference, a phenomenon seen when two or more waves interact.

Scientists from King’s College London and Universitat Politècnica de València in Spain have improved on previous cumbersome and inefficient attempts to use light to control the traveling direction of electromagnetic waves in materials. They demonstrated that circularly polarized light — light containing spinning photons — and metallic nanostructures achieve a “water wheel” effect to send lightwaves in a single direction along a metal surface. Previous attempts to produce unidirectional light have only worked using single wavelengths and have not allowed for the resultant wave’s direction to be easily switched.

Scientist at King’s College London and Universitat Politècnica de València have demonstrated that circularly polarized light and metallic nanostructures achieve a “water wheel” effect to send light waves in a single direction along a metal surface. Pictured here, a circularly polarized dipole over a metal surface, exciting surface plasmons unidirectionally. The height of the metal surface represents the surface charge density. Courtesy of Francisco J. Rodríguez-Fortuño.
“Wave interference is a basic physics phenomenon, known for many centuries, with myriad applications,” said Anatoly Zayats, a professor in the department of physics at King’s College London. “When we observed that it can lead to unidirectional guiding when spin-carrying photons are used, we could not at first believe that such a fundamental effect had been overlooked all this time.”

To illuminate a small metal structure, the researchers used circularly polarized light, which caused the electrons in the structure to move in circles, clockwise or anticlockwise depending on the direction of the photons’ spin. If the structure is brought close to an optical waveguide or a metal surface, waves in these materials moved in one selected direction only — a feat not previously achieved. Altering the light’s polarization direction can reverse the ultimate direction of the excited wave.

“We have presented an entirely new concept, surprisingly simple, that can be used as the foundation of various novel devices,” said Francisco Rodríguez Fortuño, a doctoral student at Universitat Politècnica de València and lead author of the paper, which appeared in Science (doi: 10.1126/science.1233739). “The phenomenon holds promise for spin sorting of photons, processing of polarization-encoded information and much more.”

The investigators are now working to develop applications in nanophotonics and quantum optics, Zayats said.

Related research controlling light using surface plasmon polaritons was released by Federico Capasso’s group at Harvard School of Engineering and Applied Sciences. (See: Plasmon Waves Control Light)

With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam...